Metal-Based Microchannel Heat Exchangers Made by Molding Replication and Assembly

ABSTRACT

Compression molding of metals is used to make microchannel heat exchangers. Heat transfer can be improved by employing controlled microchannel surface roughness. Flux-free bonding is achieved using a eutectic thin-film intermediate layer. Seals are leak-tight, mechanically strong, and uniform across multiple contact areas. The metal heat exchangers may be mass-produced inexpensively, and are useful for applications including the cooling of computer chips and other high-power electronic devices, air conditioning, refrigeration, condenser plates, radiators, fuel cell heat management, and instant water heating.

The development of this invention was partially funded by the UnitedStates Government under grant number CMMI-0556100 awarded by theNational Science Foundation. The United States Government has certainrights in this invention.

(In countries other than the United States:) The benefit of the 14 Jan.2008 filing date of U.S. provisional patent application 61/020,789 isclaimed under applicable treaties and conventions. (In the UnitedStates:) The benefit of the 14 Jan. 2008 filing date of U.S. provisionalpatent application 61/020,789 is claimed under 35 U.S.C. §119(e).

TECHNICAL FIELD

This invention pertains to microscale structures made by compressionmolding of high melting-temperature metals, particularly to metal-basedmicrochannel heat exchangers.

BACKGROUND ART

The nearly exponential growth in the heat generated by miniaturizedelectronic devices in recent years demands significant improvements incooling technology. Existing fan-assisted air cooling methods will beinsufficient for the next generation of microprocessors. Onlyliquid-cooled heat exchangers will be able to absorb and dissipate heatrapidly enough to maintain safe microprocessor operating temperatures. Astringent requirement of high efficiency is imposed on such a heatexchanger. The cooling system must be small and must be a closed loop,so that it may: a) fit within a desktop or laptop computer; and b) notrequire external cooling water.

Microchannel Heat Exchangers

Decreasing the liquid cooling channel dimensions to the micron scale ina solid-liquid heat exchanger leads to high heat transfer rates.Convective heat transfer from the channel surface to water is fast, butdiffusional heat transfer from the liquid at the interface to liquid inbulk is slow. By reducing the liquid cooling channel dimensions, theinterface area-to-bulk volume ratio increases, thereby reducingrate-limiting diffusional heat transport.

There have been prior demonstrations of high solid-fluid heat transferfrom microchannels, primarily in silicon-based microchannel heatexchangers. The use of silicon in such devices in these studies was notso much because silicon has desirable heat transfer properties, butrather because fabrication techniques for Si-based, high-aspect-ratiomicroscale structures (HARMS) are relatively mature and widelyavailable. Indeed, Si possesses a substantially lower bulk thermalconductivity than that of the metals that would otherwise be preferredin larger-scale heat exchangers, such as Cu and Al. Further, Si isrelatively brittle, and consequently Si-based devices tend to be fragileand easily damaged.

Si microfabrication techniques typically involve a photolithographyprocess in which a uniform, polymerizable resist layer is deposited ontoa Si substrate, and a desired pattern is photoexposed into the resistlayer. Unpolymerized resist is removed chemically or by solvation, andthe Si substrate is etched through the developed resist pattern eitherby wet chemical etching (WCE) or reactive ion etching (RIE). Additionaldeposition and etching of thin metal films may be required for the RIEprocess. Photolithography and etching are required for each Simicroscale device, and to enjoy an economy of scale, a substantialinvestment in large clean room and thin film deposition facilities isrequired.

Microchannel heat exchangers have also been fabricated in materialsother than Si by the LiGA process. LiGA combines deep X-ray/UVlithography (Lithographie) of a polymeric resist, followed by metalelectrodeposition (Galvanoformung) into the developed resist recesses toform durable, primary HARMS. Replication of secondary HARMS from theprimary HARMS via molding (Abformung) then follows. For example, U.S.Pat. No. 6,415,860 discloses Ni electrodeposition to make microscale Nimold inserts that are then used to mold microchannel heat exchangers inpolymethylmethacrylate (PMMA). Metal-based crossflow heat exchangers,such as those made from NiP alloys, were also made, by an additionalelectroless deposition onto LiGA-fabricated polymer templates. F. Ariaset al., “Fabrication of metallic heat exchangers using sacrificialpolymer mandrils,” JMEMS vol. 10, p. 107 (2001) reported the fabricationof Ni-based heat exchangers by electrodeposition of nickel ontosacrificial polymer mandrels.

There are unfilled needs in existing heat exchangers. For example, thethermal conductivity of PMMA is poor, and PMMA-based microchannel heatexchangers cannot endure temperatures higher than about 100° C. WhileNi-based and NiP-based heat exchangers can function at highertemperatures, their heat conductivities are still less than optimal.Furthermore, the electrode-based and electroless deposition techniquesused to make them are slow, and require close monitoring and control.Their cost of fabrication is high and is expected to remain high becauseof the extra deposition steps involved in these “lost-mold” processes.

Existing Si microfabrication techniques do not work for makingmetal-based microstructures. For example, the structural and chemicalisotropy of polycrystalline metals leads to removing material in asomewhat isotropic manner in a WCE process, broadening features fromthose defined lithographically. RIE techniques are also inappropriatefor metallic substrates. Because metal-based microchannel devices arehighly desirable for heat transfer applications, there is an unfilledneed for improved fabrication techniques to mass-produce metal-basedmicrochannel devices rapidly and inexpensively.

Microchannel Fabrication by Compression Molding

Microscale compression molding, or hot embossing, of polymeric plasticmaterials is an established technique. First, a primary HARMS moldinsert is produced, typically through a sequence of lithography,etching, deposition steps, with optional additional steps. Second, themold insert is impressed into a substrate, and polymer fills voids inthe mold insert through viscous or plastic flow to form the negative ofthe insert pattern. A large number of negative HARMS replicas can bereproduced from a single primary HARMS. In principle, under favorableconditions one primary mold insert may be used to produce hundreds oreven thousands of replicas rapidly and at low cost.

The quality of the replica depends upon, among other factors, themechanical yield strength of the mold insert at elevated moldingtemperatures. An important problem confronting compression metalmicrostructure molding is the lack of microstructure mold insertmaterials that retain high mechanical yield strengths at the moldingtemperatures required for metals. An electrodeposited Ni mold insert,for example, suffers permanent shape deformation when used to mold ahigher-melting temperature metal, such as Cu.

Another problem can arise from chemical reactivity between the moldinsert and the metal substrate. During compression, chemical bonds canform between the insert and the substrate. These bonds can cause theinsert to break and can damage the molded structure as it is withdrawnfrom the substrate. These surface chemistry problems had restricted themetals that could be used as mold inserts and as substrates, until thedevelopment of a conformal ceramic surface coating to inhibit chemicalbond formation. Using ceramic conformal coatings, secondary HARMS havebeen successfully reproduced in previously problematic, chemicallyreactive metals, such as Zn and Al, with LiGA-fabricated Ni moldinserts. See generally D. Cao et al., “Amorphous hydrocarbon based thinfilms for high-aspect-ratio MEMS applications,” Thin Solid Films 398-399(2001)553-559; and D. Cao et al., “Conformal deposition of Ti—C:Hcoatings over high-aspect-ratio micro-scale structures and tribologicalcharacteristics,” Thin Solid Films 429 (2003)46-54.

Bonding the Cover Plate

Once a microchannel has been fabricated in a substrate, whether Si ormetal, a leak-tight cover plate must be affixed before it can be used asa practical heat exchanger. Several bonding methods have been reportedfor Si-based microsystems, including anodic bonding and direct bonding.However, these techniques are not well-suited for bonding metal-basedHARMS.

Eutectic Bonding

Braze-bonding of bulk metal pieces has previously been used in differentapplications. Brazing is a joining process in which a non-ferrous fillermetal or alloy is heated to its melting temperature and distributedbetween two (or more) close-fitting metal parts by capillary action. Thefiller metal can optionally be a eutectic mixture. A “eutectic” mixtureis a mixture whose proportions are such that the melting point is as lowas possible; and such that the constituents of the mixture allcrystallize simultaneously at this temperature from molten liquidsolution, a temperature that is called the eutectic point. For example,it has been reported that thin films of Si, Si—Al, and Zn—Al have beendeposited onto bulk Al pieces by electron beam evaporation orsputtering. These Al pieces were then braze-bonded to one another byheating to 578-595° C., with flux introduced to remove surface aluminumoxides. This technique would be unsuitable for use with microchannels,however, because flux residue would tend to block the microchannels.

D. Tuckerman et al., “High performance heat sinking for VLSI,” IEEEElect. Dev. Lett. 2, 126-129 (1981) discloses a water-cooled, integralheat sink fabricated in silicon with a Pyrex cover plate.

A. Tiensuu et al., “Assembling three-dimensional microstructures usinggold-silicon eutectic bonding,” Sensor Actuat A 45, 227-236 (1994)discloses the use of gold-silicon eutectic bonding to join siliconmicroelements to one another.

B. Vu et al., “Patterned eutectic bonding with Al/Ge thin films formicroelectromechanical systems,” J Vac Sci Technol B 14(4):2588-2594(1996) discloses the use of an aluminum/germanium eutectic to bondsilicon dice to one another.

P. Lee et al., “Investigation of heat transfer in rectangularmicrochannels,” Int. J. Heat Mass Transf, vol. 48, no. 9, pp. 1688-1704(2005) discloses measurements and numerical modeling of heat transfer inrectangular microchannels. Test pieces were made of copper, with tenmicrochannels in parallel, and a polymeric cover plate.

D. Cao et al., “Microscale compression molding of Al with surfaceengineered LiGA inserts,” Microsyst Technol. 10 (2004) 662-670 disclosesthe use of high-temperature compression molding of aluminum plates withhigh-aspect ratio microscale mold inserts made of nickel conformallycoated with a titanium-containing hydrocarbon. See also W. Meng et al.,“Stresses during micromolding of metals at elevated temperatures: pilotexperiments and a simple model,” J. Mater. Res. 20 (2005) 161-175; J.Jiang et al., “Further experiments and modeling for microscalecompression molding of metals at elevated temperatures,” J. Mater. Res.22 (2007) 1839-1848; U.S. Pat. No. 7,114,361; and U.S. published patentapplication 2005/0056074.

F. Mei et al., “Eutectic bonding of Al-based high aspect ratiomicroscale structures,” Microsyst Technol. 13: 723-730 (published online16 Jan. 2007) reports work from our research group concerning theeutectic bonding of Al-based high aspect ratio microscale structureswith Al—Ge intermediate layers. See also F. Mei et al., “Evaluation ofeutectic bond strength and assembly of Al-based microfluidic structures,Microsyst Technol. 14: 99-107 (published online 3 Apr. 2007); F. Mei etal., “Fabrication, assembly, and testing of Cu- and Al-basedmicrochannel heat exchangers, J. Microelectromechanical Systems 17(4):869-881 (published online Jun. 27, 2008); and F. Mei et al., “Evaluationof bond quality and heat transfer of Cu-based microchannel heat exchangedevices,” J Vac Sci Technol A 26(4):798-804 (published online Jun. 30,2008).

U.S. Patent Application 2006/0142401 discloses the use of partialboiling in a minichannel or microchannel to remove heat from anexothermic process. Surface roughness was said to enhance nucleation forboiling. See, e.g., Example 11.

U.S. Patent Application 2006/0157234 discloses a microchannel heatexchanger, and briefly mentions surface roughness.

U.S. Pat. No. 5,727,618 discloses a modular microchannel heat exchangerformed from a stack of multiple thin copper sheets etched with rows ofelongated holes, coated with silver and held together with the holesaligned, e.g. with pins. The stack is heated, and the copper and silverform a fused or eutectic alloy brazing the sheets together. The holesthrough the multiple sheets then form a microchannel.

SUMMARY OF THE INVENTION

We have discovered novel metal-based microscale structures, and methodsof making those microstructures, including but not limited to structuresthat incorporate microchannels. The preferred manufacturing methodemploys compression molding of high melting-temperature metals, forexample copper, copper-based alloys, aluminum, and other metals. Moldingreplication of such metals requires mold inserts with high yieldstrengths at elevated molding temperatures. We have developed a processto fabricate mold inserts made of refractory metals that possessheretofore unattainable microscale detail.

Another aspect of the invention is the joining of microscale structuresmade of similar or dissimilar metals via a bonding process involvingthin film eutectic or near-eutectic mixtures comprising micro- ornano-domains that are substantially smaller than the contact areas ofthe microstructures to be joined.

Another aspect of the invention is the connection of compression-moldedmetal microchannels, substantially free of obstructions, to largerchannels/plena for fluidic connection to tubes, pipes, valves and otherexternal fluidic elements, where the external elements typically (butnot necessarily) have larger dimensions than the microchannels.

Yet another aspect of the invention is the modulation of microchannelwall roughness. Enhanced wall roughness is a consequence of thepreferred manufacturing methods disclosed here, but does not generallyresult from other microchannel fabrication methods. The enhancedroughness causes a surprising and significant enhancement in heattransfer from the microchannel surface to a fluid flowing through it.Without wishing to be bound by this hypothesis, we believe that theimproved heat transfer is a consequence of hydrodynamic effects inducedby the microchannel surface roughness.

We have developed improved methods for bonding metal microchannel platesto plain metal plates, other metal microchannel plates, or other metalstructures. Flux-free bonding is achieved using a eutectic thin-filmintermediate layer. The novel technique produces seals that areleak-tight, mechanically strong, and uniform across multiple microscalecontact areas. The thin film intermediate layers are fabricated, forexample, by vapor phase deposition onto the metallic HARMS or othermetallic layers to be bonded, such as by physical or chemical vapordeposition. An alternate method is to sandwich free-standing thin filmsof metals or alloys between the metallic HARMS or other metallic layersto be bonded. Metal heat exchangers in accordance with the presentinvention may be mass-produced inexpensively, and are useful fornumerous applications, including for example the cooling of computerchips and other high-power electronic devices, air conditioning,refrigeration, condenser plates, radiators, fuel cell heat management,and instant water heating. The heat exchangers are capable of handlinghigh internal pressures, e.g. 100, 200 or 300 atmospheres or evenhigher. Because they are made of metal, they are better able towithstand external mechanical stress and shock than devices made fromsilica or from polymers. They can have a very low-profile construction;for example, the distance between the heat source and the fluid mediummay be as small as 500 μm or even closer.

Using the novel technique, it is not necessary to use a large number ofthin layers to build up microchannels. Rather, the microchannels may beformed from only two metal layers, either or both of which can includeopen channels prior to their bonding together; and an intermediate layerof eutectic or eutectic precursor to braze the two metal layers to oneanother. The open channels in the metal layer or layers are preferablyformed by molding, but may also be formed by techniques otherwise knownin the art, such as milling. Depending on the geometry of the particulardevice, precise alignment between the two metal layers can sometimes beimportant, while in other instances such precise alignment may not beneeded (for example, if all open channels are in one of the two metalpieces, and if the second piece acts as a cover plate to convert theopen channels into closed channels).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) depict SEM micrographs of an example of Al moldingreplication, and the typical sub-micron sidewall roughness in thereplicated Al HARMS.

FIG. 2 depicts an SEM high magnification view of one section of a moldedmicrochannel in Al, showing elevated surface roughness on themicrochannel sidewalls.

FIG. 3 depicts an SEM micrograph of an amorphous silicon nitride-coatedInconel mold insert.

FIG. 4 depicts a high magnification SEM micrograph of an amorphoussilicon nitride coated Inconel mold insert, showing a typicalrectangular microprotrusion.

FIG. 5 depicts an EDS spectrum of an Al—Ge film that was deposited withan Al cathode current of 0.5 A and a Ge cathode current of 0.4 A.

FIG. 6 depicts the Ge to Al composition ratios for a series of Al—Gefilms that were deposited at a fixed Al cathode current of 0.5 A, as afunction of the Ge cathode current. The dashed line is an interpolationbetween the measured points.

FIG. 7 depicts a cross-sectional view of an embodiment of a typical,one-layer, Al-based microchannel structure bonded with Al—Genanocomposite thin film intermediate layers.

FIG. 8 depicts a cross-sectional view of an embodiment of a two-layer,Al-based, microchannel device bonded with Al—Ge nanocomposite thin filmintermediate layers.

FIG. 9 depicts a high magnification view of a typical bonded Almicrochannel from the device depicted in FIG. 8.

FIG. 10 depicts a cross-sectional view of a portion of a Cu-basedmicrochannel structure bonded with Al—Ge nanocomposite thin filmintermediate layers.

FIG. 11 depicts a cross-sectional view of a portion of a Cu-basedmicrochannel structure bonded with a 10 μm free-standing Al thin filmintermediate layer.

FIG. 12 depicts measured Al—Al interface bond strength as a function ofbonding temperature.

FIG. 13 depicts measured Al—Al interface bond strength as a function ofapplied pressure during bonding.

FIG. 14 depicts measured Al—Al interface bond strength as a function ofthe thickness of the nanocomposite Al—Ge thin film intermediate layerdeposited on bonding surfaces.

FIG. 15 depicts measured Cu—Cu interface tensile bond strength as afunction of the average applied pressure.

FIG. 16 depicts measured Cu—Cu interface tensile bond strength as afunction of the thickness of the free-standing Al thin film intermediatelayer.

FIG. 17 depicts three X-ray diffraction patterns obtained from threefractured Cu interfaces that had been bonded with free-standing Al thinfilm intermediate layers, 10, 25, and 38 μm thick.

FIG. 18 depicts one embodiment showing a transition from a longmeandering microchannel to a broadened outlet leading to a larger fluidplenum.

FIG. 19 depicts a molded Cu coupon containing an array of rectangularmicrochannels, a fluid supply channel, and a fluid drain plenum. Numbersshown on the ruler in the foreground are in mm.

FIG. 20 depicts a detailed view of a transition from a microchannelarray to a fluid supply channel formed by a single cut with μEDM.

FIG. 21 depicts a detailed view of a typical Cu-based microchannel arrayfabricated by molding replication.

FIG. 22 depicts Cu device surface temperature versus time as waterflowed through the microchannel array.

FIG. 23 depicts the fast time constants associated with surfacetemperature drop for Cu and Al microchannel devices.

FIG. 24 depicts a pre-assembly, two-layer, Cu-based, instant waterheater prototype. Numbers shown on the ruler in the foreground are inmm.

FIG. 25 depicts the difference in water temperature between the outletand inlet of the Cu instant water heater prototype as a function of thewater flow rate.

FIG. 26 depicts overall heat transfer efficiency of the Cu instant waterheater prototype as a function of the water flow rate.

MODES FOR CARRYING OUT THE INVENTION Microchannel Fluid Flow

X-ray/UV lithography LiGA processes can produce very smooth sidewalls inthe developed polymeric resist recesses, and the high fidelity ofelectrodeposition and molding processes conveys this smoothness to themolded metal. FIG. 1 depicts an example of Al molding replication andthe typical sub-micron sidewall roughness in the replicated Al HARMS.The novel methods allow one to control surface roughness, in particularto increase surface roughness, as compared to the surface smoothnessthat is more typically achieved by the LiGA process. We have discoveredthat, surprisingly, modulation of surface roughness can producesignificant improvements in microchannel heat exchange.

Microchannel cover plate bonding should satisfy several requirements.First, bonding across the mating surfaces for each individualmicrochannel should be complete to prevent cross-communication betweendifferent microchannels. Second, because microchannels generally havehigh internal flow resistance, they will often require significantdriving pressures, which in turn means that bond strengths should behigh. Finally, to reduce the likelihood of microchannel deformation arelatively low bonding temperature is desirable, a temperature thatdepends on the melting temperature of the particular metal. For example,with an aluminum-based device, a bonding temperature lower than about578-595° C. is desirable, since the melting temperature of aluminum is660° C. (or 933° K).

Fabrication of Metal-Based Microchannels by Compression Molding

One aspect of the invention pertains to compression molding ofmetal-based microchannels. In a preferred embodiment, mold inserts arefabricated from a refractory metal or alloy, for example Ta or thesuperalloy Inconel X750® (a nickel-chromium-iron alloy, with additionsof aluminum, titanium, and niobium). The insert may be fabricated, forexample, by using micro electrical discharge machining (μEDM) to createmicroscale recesses in a refractory metal or alloy, followed byelectrochemical polishing (ECP) to remove surface layers damaged duringμEDM. Next is an optional, but preferred conformal deposition of abond-inhibitor coating, for example as described in U.S. Pat. No.7,114,361, using, for example, a ceramic or ceramic mixture coating,such as amorphous carbon or silicon nitride. The coated, refractorymetal mold inserts may then be used to make HARMS in a softer metal,such as Cu or Ni, by molding replication with little or no apparentdamage to the mold inserts.

Significant cost benefits are achieved when a refractory mold insert canbe used to produce a large number of metal replicas. We have found thatvarious refractory metals, including for example Ta, W, Mo; refractoryalloys such as Ni-based superalloys and Fe-based tool steels; andengineering ceramics such as transition metal carbides and transitionmetal nitrides, possess sufficient mechanical yield strengths torepeatedly produce high-fidelity replicas without substantial damage ordegradation. Because these refractory metals and alloys cannotconveniently be electrodeposited using known techniques, we developed aμEDM/ECP process to fabricate mold inserts with intricate, microscalefeatures.

Specifically, we have used μEDM to create microscale trenches inrefractory metals and alloys, thereby achieving desired mold insertgeometries. The μEDM process is preferably followed by ECP to removesurface damage from the μEDM step. Importantly, control of ECP operatingparameters allows one not only to minimize surface roughness, but toalso impart roughness to the mold insert surfaces when desired. We havefound that a conformal coating deposition following ECP, which is usedto prevent chemical reactions and bonding between the mold insert andthe molded metals, generally does not substantially change the moldinsert's surface roughness. Control of the ECP parameters allows one toimpart surface roughness on the order of tens of microns, measured asthe mean peak-to-valley roughness Rz. The microchannel surface roughnessis significant. We unexpectedly discovered that increased surfaceroughness can substantially enhance the exchange of heat from themicrochannel wall to fluid flowing through the microchannel.

In one embodiment, μEDM combined with LiGA-fabricated electrodes, ECP,and conformal coating deposition can be used to create mold inserts withcomplex geometries, such as branched, serpentine, or meanderingmicrochannels, and microchannels with asymmetric cross-sections, unequaldepths, and other arbitrary profiles.

Because the molding replication process faithfully reproduces a negativeof the mold insert onto the molded metals, the resulting microchannelshave a surface roughness similar to that of the mold insert. FIG. 2depicts a portion of a single meandering microchannel in Al, created byreplication from a Ta mold insert that was fabricated by μEDM and ECP,followed by a conformal coating with Ti-containing, amorphous,hydrogenated carbon. In contrast to the smooth surfaces of theLiGA-fabricated Ni inserts, as shown in FIG. 1, the microchannelroughness depicted in FIG. 2 was five microns or higher, reflecting thatof the Ta insert.

EXAMPLE 1

Insert fabrication. Microscale mold inserts were fabricated in Ni-basedsuperalloy Inconel X750® plates in three steps: 1) μEDM of the activearea; 2) electrochemical polishing (ECP) of the machined microscaleInconel features; and 3) deposition of a conformal, amorphous siliconnitride (a-Si:N) coating over the electrochemically polished microscalefeatures. As-received Inconel plates were machined to square insertblanks, with an active area of ˜15000 μm×˜15000 μm, ˜3200 μm in height.The top surface of the blank was mechanically polished with SiC abrasivepapers down to 1200 grit size. A SARIX high precision micro erosionmachine, model SR-HPM-B, was used for insert μEDM. Flat molybdenum (Mo)sheets with a thickness of 500 μm were used as blade electrodes. Aseries of parallel cuts was made on the insert blank. Erosion of theinsert blank by μEDM produced a trench under the Mo electrode.Sequential cuts led to the formation of an array of trenches, or anarray of parallel rectangular microprotrusions between trenches.As-machined Inconel blanks were electrochemically polished for 10 min in“current-controlled” mode in a mixed acid solution of HClO4 (70%) andCH3COOH (80%) at a volume ratio of 1:1. Following ECP, a conformala-Si:N coating was deposited over the Inconel inserts in a radiofrequency (rf), inductively coupled plasma (ICP)-assisted, hybridchemical/physical vapor deposition system, generally following theprocedures of W. J. Meng et al., “Temperature dependence of inductivelycoupled plasma assisted deposition of titanium nitride coatings,” Surf.Coat. Technol. 120/121, 206 (1999).

Surface morphology was examined both with a Hitachi S3600N scanningelectron microscope (SEM), and with a Veeco Wyko3100 opticalprofilometer (OP). An SEM overview of one a-Si:N coated Inconel insertis shown in FIG. 3. The thickness of the a-Si:N coating was ˜600 nm. Theinsert contained 19 parallel rectangular microprotrusions, withcenter-to-center spacings of ˜750 μm. The sequential cutting process ledto variations in the widths of the microprotrusions. The average widthof all microprotrusions, as measured from the SEM micrographs, was 154μm. The trench bottoms were somewhat rounded. The height of the straightsection of the microprotrusions was ˜400 μm. FIG. 4 depicts a close-upSEM image of a typical microprotrusion. The numerous micron-scalefeatures on the microprotrusion surface resulted from preferentialetching during the ECP process, probably delineated by the grainstructure of the underlying Inconel substrate. The a-Si:N coatingpresumably adheres to the substrate morphology conformally withoutsignificantly modifying it.

EXAMPLES 2 AND 3

Microchannel compression molding in Cu and Al using refractory moldinserts with enhanced surface roughness. Cu 110 (99.9+wt. % Cu) and Al6061 (1.0 wt. % Mg, 0.6 wt. % Si, 0.27 wt. % Cu, 0.2 wt. % Cr, balanceAl) coupons, with the same geometry of 35.5 mm×35.5 mm×6.4 mm, weremolded at high temperatures with the a-Si:N coated Inconel insert shownin FIGS. 3 and 4. Before molding, the top surfaces of the Cu and Alcoupons were mechanically polished with SiC abrasive papers of differentsizes and finished with a 1 μm diamond particle suspension. Compressionmolding was carried out in a MTS858 single-axis testing systeminterfaced to a high-vacuum molding chamber, which housed two heatingstations. One heating station was mechanically attached to the bottom ofthe vacuum chamber. A second heating station was mechanically attachedto the top linear actuator of the MTS858 testing system through abellow-sealed motion feed-through. The two heating stations were heatedseparately by resistive heating cartridges, and temperatures weremonitored by two separate K-type thermocouples. The coupon to be moldedwas fastened to the top surface of the bottom heating station, while theinsert was mechanically attached to the bottom surface of the topheating station, which in turn was attached to the linear actuator.Total axial force and axial displacement of the insert were measuredcontinuously during the entire molding and demolding process. Molding ofCu coupons occurred with both the coupon and the insert heated to ˜500°C., while molding of Al coupons occurred with both the coupon and theinsert heated to ˜400° C. The molding process faithfully replicated thearray of 19 parallel rectangular microprotrusions into parallelrectangular microchannels in both the Cu and the Al coupons. The averagewidth of microchannels on the Cu coupon was measured as 150 μm. Theaverage width of microchannels on the Al coupon was measured as 148 μm.The final width of the molded microchannel tended to be somewhat smallerthan that of the corresponding microprotrusion on the mold insert, by˜3% for Cu, and ˜4% for Al. The depths of the molded microchannels weregreater than 400 μm.

Optical profilometry (OP) images from the bottoms of replicated Cumicrochannels were used to assess the surface roughness within moldedmicrochannels, the peak-to-valley roughness Rz. (Data not shown; seeFIG. 5 of priority application 61/020,789.) Four independent OP imageswere obtained from the bottom surfaces of four different microchannelsin the Cu and Al coupons. Values of Rz obtained from the four OP imageswere averaged. The average Rz values measured for the bottom surfaces ofthe microchannels were 11.8 μm for Cu, and 8.2 μm for Al. Due todifficulties of optical access, values of Rz for the microchannelsidewalls were not obtained. From qualitative SEM observations, it issurmised that surface roughness of the microchannel sidewalls wassomewhat smaller, about 5 μm.

As expected from the replication process, surface roughness on themicrochannel bottoms mimicked the roughness of the top surfaces of theinsert microprotrusions. The surface roughness on the mold insert isbelieved to result from preferential etching during the ECP step ofinsert fabrication. The observed Rz values, about 10 μm, were largerthan what is typically obtained from mechanical micromilling of metals,and greatly exceeded what is typically obtained from electrodepositedmetallic structures formed by LiGA processing.

Eutectic Bonding of Metal-Based Microchannel Structures Using Thin FilmIntermediate Layers

Another aspect of this invention pertains to joining two or moremetallic structural components, one or both of which may containmicrochannel features, using one or more thin film intermediate layers.The problem solved was to create uniform, mechanically strong bondsacross numerous mating surfaces, of microscale dimensions, of possiblydissimilar metals, without requiring flux.

A preferred, novel technique employs flux-less, eutectic bonding. Themetal components to be bonded were first subjected to plasma etching invacuum, followed by deposition of an Al—Ge eutectic nanocomposite thinfilm. The plasma etch removed surface metals oxide, removing any needfor flux. With small channel dimensions, the thickness of the eutecticnanocomposite intermediate layer should also be small to avoidmicrochannel blockage, typically ranging from below 1 micron to about 10microns. The eutectic intermediate layer is used to ensure that thebonding interface will melt at a substantially lower temperature thanthe melting temperature of the metal components to be joined, therebyminimizing mechanical deformation at the temperature and pressure usedfor bonding. For example, we have successfully bonded Al-basedmicrochannel structures in the temperature range 450° C. to 550° C.,considerably below the 660° C. melting point of elemental aluminum.

Eutectic mixtures are generally not homogeneous, but are insteadtypified by a collection of domains, each of which consists primarily ofone of the eutectic's elemental components. The domain size effectivelydictates a lower limit on the size of areas to be joined by eutecticbonding. Indeed, it is preferred that the eutectic domain size should besubstantially smaller than the dimensions of the areas to be bonded. Forexample, with an Al—Ge eutectic nanocomposite thin film, typical domainsizes are on the order of 100-200 nm. These small domains not onlyensure uniform, strong bonding across areas that can have relativelysmall dimensions, but also ensure that melting across the entire bondinginterface occurs at or near the lowest possible temperature (i.e., at ornear the eutectic point), reducing the potential for mechanicaldeformation of the microchannels or other structures during bonding. Theeutectic thin film intermediate layers may be fabricated, for example,by vapor phase deposition methods, such as physical or chemical vapordeposition onto metallic HARMS, or by sandwiching free-standing thinfilms of metals or alloys between the metallic pieces to be bonded. Themelting point of the eutectic or near-eutectic thin film should besubstantially below the melting point of the components to be bonded,preferably at least about 50° C.-100° C. lower.

The nanodomain eutectic bonding technique is not limited toplate-to-plate bonding. It may be used for other metal-to-metal bonding,for example to elaborate a plain or molded metal surface by serialaddition of semi-layers, rods, alignment pins, and the like.

Bonding of Al-Based Microchannel Structures with Al—Ge NanocompositeThin Film Intermediate Layers.

EXAMPLE 4

Synthesis of Al—Ge nanocomposite thin films, and bonding of one-layerdevices. Al—Ge composite thin films were deposited with a radiofrequency (rf) inductively coupled plasma (ICP)-assisted hybrid tool,which combined a 13.56 MHz ICP with direct current (dc) balancedmagnetron sputtering. One Al (99.99%) cathode and one Ge (99.99%)cathode, each 75 mm in diameter, were placed facing each other and weresputtered in current-controlled mode. The distance between the Al and Gecathodes was about 250 mm. Substrates for Al—Ge deposition included Siwafers with diameter 50 mm, polished Al coupons (Al 1100, 99%+) withdiameter 35 mm, and replicated Al HARMS (Al 1100, 99%+). The substrateswere ultrasonically cleaned in acetone and methanol before being mountedon a rotatable holder between the two targets. The ultimate basepressure of the deposition system was less than 1.0×10⁻⁸ Torr. Typicalbackground pressures prior to deposition runs were about 1×10⁻⁷ Torr.

The entire deposition sequence was carried out in an Ar (99.999%)atmosphere, with a total pressure of about 1.3 mTorr. The depositionsequence comprised a substrate surface etch, followed by codeposition ofAl and Ge. Substrate etching occurred in a pure Ar ICP with a totalinput rf power of 1000 W, a substrate bias of −200 V, and an etchduration of 3 min. Sputtering of Al and Ge cathodes commencedimmediately after substrate surface etch.

One series of Al—Ge composite films was deposited on Si(100) substrates.For this series of specimens, a fixed Al cathode current of 0.5 A wasused, and the Ge cathode current was varied from 0.1 A to 0.5 A to alterthe Ge composition within the film. The deposition duration was fixed at30 min for this series of specimens. Additional depositions on flat Alcoupons and replicated Al HARMS were carried out with a fixed Al cathodecurrent of 1.0 A and a fixed Ge cathode current of 0.45 A for 60 min.The substrate bias was fixed at −100 V for all depositions. Substrateswere rotated continuously in the center of the deposition zone at about12 rpm during both etching and deposition. No intentional substrateheating or cooling was applied during the deposition process.

Bonding experiments were carried out using a MTS858 single-axis testingsystem interfaced to a custom-built high-vacuum chamber. Aturbomolecular pump system produced an ultimate molding chamber basepressure of about 2×10⁻⁹ Torr Typical background pressures duringbonding experiments were about 1×10⁻⁶ Torr. Two heating stations wereinstalled in the vacuum chamber. The lower heating station wasmechanically attached to the bottom of the vacuum chamber. The upperheating station was connected to the linear actuator through abellow-sealed motion feedthrough. The two heating stations wereseparately heated by resistive heating cartridges, and the temperatureswere measured by two separate K-type thermocouples. Careful machining ofthe heating stations ensured that the surfaces of the two heatingstations were parallel to each other and perpendicular to the actuatoraxis. The linear actuator could be programmed to move either accordingto prescribed load forces in a force-controlled mode, or according toprescribed actuator displacements in a displacement-controlled mode. Thetotal axial force was measured by a 25 kN load cell, and the total axialdisplacement of the actuator was measured by a linear variabledisplacement transducer. Flat Al coupons and Al HARMS with Al—Gecomposite films deposited on the bonding surfaces were placedface-to-face on the lower heating station. The chamber was evacuated,and both heating stations were heated to about 400° C. The upper heatingstation was then contacted with the Al assembly to be bonded, and anincreasing compression force was applied to the assembly at a constantloading rate of 100 N/min. A constant force was held for 12 min afterthe compression force reached the desired level. The temperatures ofboth heating stations were increased during the compression forceincrease, so that the specimen temperature reached about 500° C. duringthe constant force hold. After the constant force hold, the linearactuator was withdrawn from the Al assembly and the system was allowedto cool down.

EXAMPLES 5 AND 6

Characterization of Al—Ge nanocomposite thin films and bonding oftwo-layer devices. A similar bonding process was used to assemble anAl-based, two-layer, microchannel device. Al—Ge composite thin films,with an approximate thickness of ˜2 μm, were deposited on both sides ofa polished Al foil and the feature surfaces of two replicated Al HARMS.After Al—Ge film deposition, the two Al HARMS were placed face to faceon the bottom heating station, with the Al foil inserted in the middle.Both heating stations were heated to slightly above 500° C., and theupper heating station was then placed in contact with the assembly. Anincreasing compression force was applied to the assembly at a constantloading of 300 N/min. A constant force was held for 10 min after thecompression force reached ˜1000 N, corresponding to an applied pressureof ˜1.5 MPa. After the constant force hold, the linear actuator waswithdrawn from the Al assembly and the system was cooled down.

A JEOL2010 transmission electron microscope (TEM) was used tocharacterize the micro- and nano-scale structure of the Al—Ge compositefilms. Cross-sectional TEM specimens of Al—Ge films deposited on Si(100)substrates were prepared with standard face-to-face gluing, mechanicalthinning, dimple grinding, and ion milling using 4 kV Ar⁺ ions at a 4°take-off angle on a Gatan Precision system. Compositional analysis ofAl—Ge composite thin films was performed by energy dispersive X-rayspectroscopy (EDS). EDS measurements were made using a EDAX systemequipped with an ultra-thin window detector, attached to a HitachiS3600N scanning electron microscope (SEM). EDS spectra were collected atan electron beam energy of 15 keV and a detector take-off angle of 36°.SEM examinations of bonded Al assemblies were made on a Hitachi S3600NSEM.

For eutectic bonding with an Al—Ge interlayer, the composition of Al—Gefilms was measured and correlated to the deposition conditions. Inaddition, the micro- and nano-scale structure of the Al—Ge films wascharacterized. FIG. 5 depicts an EDS spectrum collected from one Al—Gefilm, deposited at Al and Ge cathode currents of 0.5 A and 0.4 A,respectively. Similar EDS spectra were collected from Al—Ge filmsdeposited at different Al and Ge cathode currents, at the samespectrometer settings, the same magnification, and from an identicalarea about 10 μm×10 μm. The manufacturer-supplied EDS data analysissoftware, EDAX Genesis (version 3.6), was used for automatic backgroundfitting and removal. A standardless quantification routine employing ZAFcorrections was included as a part of the data analysis software, andwas used to obtain atomic percentages from raw EDS spectra. Spectracollected from Al—Ge films contained signals from Al and Ge within thefilm, as well as signal from the Si substrate. No oxygen signal was seenabove the noise level. Because there was a strong signal from the Sisubstrate, only the ratio of Ge at. % composition to Al at. %composition was considered. FIG. 6 depicts the Ge to Al compositionratio for a series of Al—Ge films as a function of Ge cathode current,deposited at a fixed Al cathode current of 0.5 A. The thickness of thisseries of Al—Ge films, as measured by cross-sectional SEM, was about 360nm. The deposition rate for this series of Al—Ge films was thus about 12nm/min. As expected, the Ge composition increased with increasing Gecathode current. The Ge to Al composition ratio for the film depositedat an Al cathode current of 0.5 A and a Ge cathode current of 0.2 A wasclose to the eutectic Al₇₀Ge₃₀ composition.

Cross-sectional TEM bright-field (BF) micrographs were taken of a filmdeposited at an Al cathode current of 0.5 A and a Ge cathode current of0.3 A. The nanoscale structure of the Al—Ge films could be seen, fromregions close to the Si(100) substrate to regions close to the topsurface of the film. (Data not shown; see FIG. 8 of priority application61/020,789.) High-resolution imaging showed that the region immediatelyadjacent to the Si(100) substrate, with dark contrast, was crystallineGe, while regions with light contrast above the Ge layer werecrystalline Al. These imaging results were also confirmed with a seriesof EDS spectra collected from the different regions. The bright-fieldmicrographs provided strong evidence of phase separation within theco-deposited Al—Ge film. In companion dark-field (DF) micrographs, theGe regions exhibited light contrast while the Al regions showed darkcontrast. The dark-field micrographs again showed that a crystalline Gelayer formed next to the Si(100) substrate. (Data not shown; see FIG. 9of priority application 61/020,789.) Immediately above the Ge layer,lateral separation occurred between the crystalline Ge and crystallineAl grains, roughly in a plane parallel to the substrate surface. As theAl—Ge film deposition continued, some Al grains began to spreadlaterally and cover the Ge region underneath, showing evidence oftransverse separation between crystalline Ge and crystalline Al grainsin the direction perpendicular to the substrate surface. Thus we sawevidence of both lateral and transverse separations between Ge and Alwithin the co-deposited Al—Ge film. A high-resolution micrograph of aco-deposited Al—Ge film with a composition close to eutectic clearlyshowed phase-separated Al-rich and Ge domains, typically several tens ofnm in size (data not shown).

Without wishing to be bound by this hypothesis, it is believed thatnanoscale separation between the crystalline Ge and crystalline Aldomains within codeposited Al—Ge films resulted from competition betweenthe thermodynamic driving force for Al—Ge phase separation and thegrowth kinetics dictated by the film deposition rate. For the purpose ofusing Al—Ge films for intermediate layer bonding, the Al and Ge domainsin such codeposited films are on the order of 100 nm or smaller. Thisintimate mixing promotes eutectic melting of the entire Al—Ge film oncethe eutectic point T_(E) is reached, and is beneficial for bonding ofmating surfaces with microscale dimensions. Codeposition of Al—Ge filmsis a preferred route for making nanoscale phase-separated Al—Ge eutecticmixtures since it can produce an intimate mixture of Al and Ge domainswith controlled phase separation. The film composition may be controlledby adjusting the individual Al and Ge sputter erosion rates.

In one embodiment, nanocomposite Al—Ge intermediate layers werecodeposited onto a replicated Al HARMS with a parallel array of straightmicrochannels, and one flat Al coupon. To test the feasibility ofsimultaneously bonding features of different sizes, the microchannelwidth was varied between 120 μm and 180 μm. The Al—Ge intermediate layerwas deposited at a fixed Al cathode current of 1.0 A and a fixed Gecathode current of 0.45 A, making the film's composition close toeutectic. After the Al—Ge intermediate layers had been deposited, thereplicated Al HARMS and the flat Al coupon were bonded at a finalholding temperature of 510° C. The applied pressure during the finalhold stage of bonding was about 1.5 MPa. FIG. 7 depicts across-sectional view of a typical, one-layer, Al-based microchannelstructure following such bonding. The replicated Al microchannelstructure and the flat Al coupon were bonded tightly together, with noevident gaps.

In another embodiment, two Al HARMS pieces were used to assemble atwo-layer structure with closed microchannels. In each Al piece, a setof parallel rectangular microchannels, ˜1 cm long and ˜330 μm deep, wasreplicated in the Al bulk from surface-engineered Inconel X750 insertsby compression molding. Two plena were machined into the Al bulk andconnected to the two ends of the microchannel array. To test thefeasibility of simultaneously bonding microfeatures of different sizes,the widths of microchannels were varied from less than 80 μm to morethan 250 μm. A polished Al foil, with an Al—Ge film deposited on bothsides, was inserted between the two Al HARMS pieces, and the bondingprocess as described above was used to produce a three-piece Alassembly, containing two layers of parallel microchannels. Holes weredrilled through the entire bonded specimen at the plenum regions on eachside of the microchannels. Threaded holes were tapped into the Al bulkfor external fluid connections. FIG. 8 depicts a cross-sectional view ofa portion of the assembled two-layer microchannel structure, obtained bymechanical cutting. The structure contained two layers, with 20microchannels in each layer. FIG. 9 depicts a close-up view of a typicalbonded microchannel. The bonding interface is not even discernable inthis high-magnification view, indicating the quality of bondingachieved.

Water was fed into one plenum, and it then flowed freely out of the cutcross-section as individual jets. (Not shown; see FIG. 13 of priorityapplication 61/020,789.)

The replicated Cu and Al microchannel arrays each contained 19rectangular microchannels. Our observations showed exit water jets fromall 19 microchannels in the assembled Cu MHE, while exit water jets wereobserved from only 18 microchannels in the assembled prototype Al MHE.It appeared that one microchannel in the assembled Al MHE had beenblocked during the bonding process; the reason for the blockage is notunderstood. Our measurements indicated that the average width ofmicrochannels in the Cu prototype device exhibited little change fromthat on the as-replicated Cu coupon, while the average microchannelwidth of the Al device had decreased somewhat from that on theas-replicated Al coupon. Considering that Al bonding occurred at about83% of the Al melting temperature, we speculate that the observednarrowing of the microchannels may have resulted from plasticdeformation during the bonding process.

EXAMPLES 7 AND 8

Bonding of Cu-based microchannel structures with Al—Ge nanocompositethin film intermediate layers and free-standing Al thin films. Toprepare prototype embodiments of one-layer, enclosed, Cu-basedmicrochannel structures, Cu coupons containing replicated microchannelswere bonded onto a flat Cu plate. The thickness of both the flat plateand the Cu coupons was 6.4 mm. The bonding surfaces of the coupons andflat plates were mechanically polished with SiC abrasive papers, andfinished with a 1 μm diamond particle suspension. Al—Ge composite thinfilm intermediate layers were deposited onto the polished surfaces bysputter co-deposition in a pure Ar (99.999%) atmosphere, at a pressureof ˜1.3 mTorr. Two separate sputter targets were used, one for pure Al(99.99%) and the other for pure Ge (99.99%). The polished Cu coupons andplates were ultrasonically cleaned in acetone and methanol before beingmounted on a rotatable holder in the middle of the deposition zone. Thedeposition sequence comprised a radio frequency (rf) inductively coupledplasma (ICP) substrate surface etch, followed by Al and Geco-deposition. Substrate etching occurred in a pure Ar ICP with a totalrf input power of 1000 W, a substrate bias of −100 V, and an etchduration of 20 min. Sputtering of Al and Ge targets commencedimmediately after the substrate surface etch. Substrates were rotatedcontinuously at ˜12 rpm during both etching and deposition. All Al—Gedepositions were carried out using fixed target currents: 1.0 A for Al,and 0.45 A for Ge. The substrate bias during deposition was held at −50V. These deposition parameters resulted in a composition ratio close tothe Al₇₀Ge₃₀ eutectic. The deposition duration was 60 min, producing anAl—Ge film thickness of ˜2 μm.

Bonding experiments were carried out using the MTS858 single-axistesting system interfaced to a high-vacuum chamber containing twoheating stations. Cu coupons containing microscale features and flat Cuplates, with Al—Ge composite films deposited on the bonding surfaces,were placed face-to-face on the lower heating station. The chamber wasevacuated, and both heating stations were heated. Bonding of the Cucoupon and plate occurred at a temperature about 540° C. with an appliedpressure about 3 MPa. FIG. 10 depicts a cross-sectional view of aportion of a prototype Cu microchannel structure bonded with Al—Genanocomposite thin film intermediate layers.

In an alternative embodiment, Cu microchannel structures were bondedwith a single free-standing Al thin film as the intermediate layer. Toform Cu-based, single-layer, microchannel structures, one Cu couponcontaining a parallel array of replicated microchannels and one blank Cucoupon were placed face to face on the bottom heating station, with a 10μm thick Al free-standing thin film inserted in the middle. Surfaces ofCu coupons were polished with 1200-grit silicon carbide papers prior tobonding. The entire assembly was placed on top of the bottom heatingstation. After evacuation, both heating stations were heated above 500°C., and the upper heating station was then put into contact with theassembly. An increasing compression force was applied to the assembly ata constant loading rate of 300 N/min. The force was held constant for 15min after the compression force had reached the desired level of ˜3000N, corresponding to an average applied pressure of about 3 MPa. Thefinal bonding temperature for the coupon/block/coupon assemblies washeld at about 580° C. After the constant force hold, the linear actuatorwas withdrawn from the assembly and the system was cooled down. FIG. 11depicts a cross-sectional view of a portion of one Cu microchannelstructure bonded with one free-standing Al thin film intermediate layer.Comparing FIGS. 10 and 11 shows that both bonding approaches producedclean, enclosed microchannel structures without blockages.

EXAMPLES 9 AND 10

Evaluation of the strength of bonded Al—Al and Cu—Cu interfaces. Tensiletesting specimens were prepared to evaluate the strength of Al—Alinterfaces bonded with Al—Ge eutectic nanocomposite thin filmintermediate layers. Al—Ge composite films were deposited onto thebonding surfaces of two cuboid Al coupons, ˜22 mm×˜22 mm in area. Thetwo Al coupons were placed face-to-face on the bottom heating station,forming an assembly ˜36 mm long in the direction perpendicular to thebonding interface. A small hole was drilled at the corner of the bottomAl coupon close to the bonding interface. A K-type thermocouple wasinserted into the hole to measure the temperature of the interfaceduring bonding. After the chamber was evacuated, both heating stationswere heated to temperatures about 10 degrees higher than the targetbonding temperatures, which were 450, 500, and 550° C. for differenttests. During the heating process, the top heating station was heldclose to the top surface of the Al coupon assembly but was not incontact with it. The temperature of the bonding interface during thisinitial heating process, as measured by the K-type thermocouple, wasalways less than 390° C., below the Al—Ge eutectic temperature of 424°C. Once the bottom and top heating stations had reached steady statetemperatures, the upper heating station was put into contact with the Alcoupon assembly using the linear actuator. A linearly increasingcompression force was applied to the Al coupon assembly such thatloading force levels of 250, 500, and 750 N were reached after aconstant duration of 8 min. These loading forces corresponded to appliedpressures of ˜0.5, ˜1.0, and ˜1.5 MPa, respectively. A constant forcewas held for 10 min after the compression force reached the desiredlevels. During the compression force increase, the temperature of thebonding interface increased to close to the target temperature of 450,500, or 550° C. Further temperature increases were registered on theK-type thermocouple during the constant force hold. For all bondingruns, the total temperature change measured by the K-type thermocoupleduring the 10 min constant force hold was less than 12° C. The nominalbonding temperature was taken to be the average value of the K-typethermocouple reading during the 10 min constant force hold. After thishold period, the heaters were turned off to cool the bonded specimen,with the same force still applied. After the temperature of the bondinginterface had decreased below 400° C., the linear actuator was withdrawnfrom the Al assembly, and the system was cooled down to roomtemperature.

We prepared a series of two Al-coupon assemblies bonded at differenttemperatures, different applied pressures, and different thicknesses ofAl—Ge intermediate layers. Using a Struers Accutom5 cutting machine, thefour outermost sections along the axial direction, each ˜3-4 mm thick,were removed from the bonded Al specimen. From the remaining specimen,which was ˜14 mm×˜14 mm×˜36 mm, four tensile testing specimens wereobtained with two perpendicular, bisecting cuts parallel to the axialdirection, resulting in final tensile specimens ˜7 mm×˜7 mm×˜36 mm.After cutting, the four surfaces of the tensile specimen parallel to theaxial direction were mechanically polished with 600 grit silicon carbidepapers. Tensile testing was conducted along the specimen's axialdirection to evaluate the tensile strength of the bonding interface,using a MTS810 system with hydraulic grips. Surface morphologies of thefractured bonding surfaces were examined on a Hitachi S3600N scanningelectron microscope (SEM), as well as with an X-ray dispersivespectroscopy (EDS) system (EDAX) equipped with an ultra-thin windowdetector.

FIG. 12 summarizes the results of our measurements of bond strength as afunction of bonding temperature. For this series of measurements, allbonding runs were conducted at an applied pressure of ˜1.5 MPa, with a˜2 μm Al—Ge intermediate layer deposited on each bonding surface (i.e.,total thickness of both Al—Ge intermediate layers at the bondinginterface was ˜4 μm). Of 15 tensile tests total, the four performed onspecimens corresponding to a bonding temperature of ˜450° C. allresulted in clean breaks at the bonding interface (denoted “break” inthe figure). Of the seven tests performed on specimens corresponding toa bonding temperature of ˜500° C., only one resulted in breaking at thebonding interface. The other six tests resulted in deformations at thegripped sections without any break at the bonding interface (denoted “nobreak” in the figure). All four tests performed on specimenscorresponding to a bonding temperature of ˜550° C. resulted indeformations at the gripped sections only (denoted “no break”). It isevident from FIG. 12 that the measured values of bond strengthsexhibited significant scatter. At a bonding temperature of ˜450° C.,measured bond strength varied from ˜85 to ˜156 MPa; and at a bondingtemperature of ˜500° C., measured bond strength varied from ˜77 togreater than ˜167 MPa.

FIG. 13 summarizes results of bond strength measurements as a functionof the pressure that was applied during bonding. For this series ofmeasurements, all bonding runs were performed at a bonding temperatureof ˜500° C., with a ˜2 μm Al—Ge intermediate layer deposited on eachbonding surface (i.e., total thickness of both Al—Ge intermediate layersat the bonding interface was ˜4 μm). At the lowest applied pressure of˜0.5 MPa, all four tensile tests resulted in clean breaks at the bondinginterface, yielding measured values of bond strength from ˜84 to ˜140MPa. Of 11 tensile tests performed on specimens bonded at appliedpressures of ˜1.0 or ˜1.5 MPa, only two tests resulted in breaks at thebonding interface. The remaining specimens failed at the grippedsections during testing. Of these tests, the highest strength valuesobtained were higher than 175 and 167 MPa, at applied pressures of ˜1.0and ˜1.5 MPa, respectively.

FIG. 14 summarizes results of bond strength measurements as a functionof the thickness of the Al—Ge intermediate layer. For this series ofmeasurements, all bonding runs were performed at a bonding temperatureof ˜500° C. and an applied pressure of ˜1.5 MPa. At the smallest Al—Gefilm thickness of ˜0.5 μm, measured bond strengths ranged from 89 toover 152 MPa. No gross differences were discerned in tests performed onspecimens bonded at Al—Ge film thicknesses of ˜1.0 or ˜2.0 μm, with thehighest measured bond strengths exceeding 167 MPa in both cases. Themaximum observed strength was obtained at an Al—Ge film thickness of˜1.0 μm, exceeding ˜188 MPa. At the three Al—Ge film thicknesses of˜0.5, ˜1.0, and ˜2.0 μm, a similar scatter in bond strengths wasobserved. The data shown in FIGS. 12-14 indicated that, notwithstandingthe scatter in the measurements, interfacial strength values exceeded˜77 MPa, or ˜770 atmospheres, under all bonding conditions tested.

The structures of the Al—Al interfaces within the bonded Al microchannelstructures were examined by combining SEM with focused ion beam (FIB)images (data not shown). The imaged area, approximately 250 μm×190 μm,was close to one corner of a microchannel. Prior to SE image and EDSspectrum acquisition, the entire area was lightly etched with a Ga⁺ ionbeam. The secondary electron (SE) image showed a band with speckledcontrast around the location of the original bonding surfaces, with awidth of ˜100 μm. The speckling apparently arose from Ge precipitates,as it was generally in the same locations corresponding to a Ge-L X-rayintensity image (data not shown). The Ge precipitates ranged in sizefrom <1 to ˜4 μm. A band of Ge precipitates also surrounded the sidewallof the Al microchannel, where the mating Al plate was not present. TheAl—Ge thin film had been deposited onto the microchannel sidewall duringsputter-codeposition, which likely accounts for the presence of Geprecipitates around the sidewall of the Al microchannel.

To confirm that the Ge precipitate band was not an artifact of themechanical polishing process, a perpendicular cut, ˜38 μm long and ˜20μm deep, was made with a focused Ga⁺ ion beam into the cross sectionsurface. This cut straddled the location of the original bondingsurfaces. Ge precipitates, ranging from ˜0.7 μm to ˜4 μm, were seendispersed within Al grains 10 μm and larger in an apparently randomfashion (data not shown). The presence of Ge precipitates was alsoconfirmed by corresponding Al—K and Ge-L X-ray intensity maps (data notshown). Other than the presence of the random Ge precipitates, no cleardemarcation could be seen to indicate the location of the bondingsurfaces of the two original Al pieces. Our observations indicated thatthe Ge precipitates were distributed in an approximately uniform manneracross an extended interface region ˜100 μm wide. SE images, and Al—Kand Ge-L X-ray intensity maps also showed the presence of a band of Geprecipitates surrounding the location of the original bonding surfaces,with a width of ˜100 μm.

Consistent observations were made in tensile fracture surfaces of bondedAl—Al specimens. For example, in an SE image of a tensile fracturesurface of a specimen bonded at ˜500° C., faceted regions with sizesranging from <1 to ˜4 μm were interdispersed with regions containingnumerous micron and submicron sized dimples (data not shown). Thechemical compositions of the faceted and dimpled regions were probed byEDS mapping. Faceted regions yielded low Al—K and high Ge-L counts,representing exposed Ge crystallites. Dimpled regions had low Ge-Lcounts, and represented the Al matrix. Our SEM fractography measurementsconfirmed Al—Ge phase separation within the bonding interface region.The presence of faceted Ge crystallites on the fracture surfacesuggested that separations occurred either at interfaces between Gecrystallites and the Al matrix, or across Ge crystallites in a brittlemanner; while the observation of micro/nano scale dimples on thefracture surface suggested that separation of the Al regions involvedductile fracture.

Without wishing to be bound by this hypothesis, we believe that theseobservations support our proposed explanation for the high averagetensile strength measured at the bonded Al—Al interface. The Al—Geintermediate layer bonding process effectively joined the Al coupon withthe flat Al plate across an extended interface region, with nodemarcation at the location of the original bonding surfaces. Theaverage tensile bond strength was dominated by ductile fracture of theAl matrix within this extended interface region. The approximatelyuniform distribution of Ge precipitates, <4 μm in size, within thisextended interface region ˜100 μm wide meant that the region of Geprecipitates across the cross section was lower than would be the casewhere all Ge atoms concentrated within a narrow interface region. Thisdispersal of the Ge precipitates increased the fractional area of Al—Albonds, and thus the average tensile strength.

Without wishing to be bound by this hypothesis, we believe that certainmechanisms likely predominated throughout the extended bonding interfaceregion. During bonding, the two solid Al pieces were in contact with alayer of Al—Ge eutectic liquid, ˜4 μm thick. Because bonding occurred at˜500° C., above T_(E)=424° C., there was likely some broadening of theliquid layer by dissolution of solid Al into the Al—Ge eutectic liquid.Simple calculations suggested that the composition shift likelybroadened the Al—Ge liquid layer from ˜4 to ˜6 μm. Furthermore, thesolidus reaction and Ge diffusion into the solid aluminum broadened thebonding interface region to ˜50 μm. Additional cross-sectionalmetallographic observations around the bonding interface region showedAl grains spanning the entire bonding interface region, suggesting thepossibility of epitaxial re-growth from un-melted Al grains.

Without wishing to be bound by this hypothesis, we believe that thenanoscale domain size of the Al-rich and Ge regions within the Al—Geintermediate layer, together with eutectic melting, aids the broadeningof the bonding interface region, and improves the bonding of microscaleAl-based structures. Generally similar results are expected using othereutectic bonding intermediates.

Tensile testing specimens were prepared to evaluate the strength ofCu—Cu interfaces bonded with an Al thin film intermediate layer. Tworectangular Cu coupons (˜25 mm×˜16 mm×˜16 mm), with surfacesmechanically polished to less than 1 μm roughness, were bonded with oneAl thin film intermediate layer. The two Cu coupons were placed face toface on the bottom heating station with a thin Al film (Al 1100, 99%+)inserted in the middle, forming an assembly ˜32 mm long in the axialdirection, perpendicular to the bonding interface. A small hole wasdrilled at the corner of the bottom Cu coupon close to the bondinginterface, into which a K-type thermocouple was inserted. After thechamber was evacuated, both heating stations were heated. The topheating station was close to the top surface of the Cu coupon assembly,but not in contact with it. After the bottom and top heating stationsreached 500° C., the upper heating station was placed in contact withthe Cu coupon assembly using the linear actuator. A linearly increasingcompression force was applied to the Cu coupon assembly, so that loadingforce levels of 425, 850, and 1700 N were reached after a constantduration of 10 min. These compressive loading forces corresponded toaverage applied pressures of ˜1, ˜2, and ˜4 MPa, respectively. Aconstant force hold was executed after the compression force reached thedesired level. During the compression force increase, the temperaturesof the bottom and top heaters were raised, leading to furthertemperature increase at the bonding interface. The interface temperaturewas recorded continuously during the constant force hold, 12 min afterthe interface temperature reached ˜550° C., corresponding to the Al—Cueutectic temperature of 548° C. During this 12 min hold, the interfacetemperature increased further and reached steady state. This steadystate temperature, as measured by the K-type thermocouple, was taken asthe nominal bonding temperature. After the 12 min hold, the heaters wereturned off with the same force still applied. After the temperature ofthe bonding interface decreased to <450° C., the linear actuator waswithdrawn from the top surface of the Cu assembly, and the system wascooled to room temperature.

A series of Cu two-coupon assemblies was bonded at different appliedpressures and with different thicknesses of Al films. Using a StruersAccutom5 precision cutting machine, the four outermost sections alongthe axial direction, ˜2-3 mm thick, were removed from the bonded Cuspecimen. From the remaining specimen, ˜20 mm×˜12 mm×˜32 mm, fourtensile testing specimens were obtained by making two perpendicularbisecting cuts parallel to the axial direction, resulting in a finaltensile specimens ˜10 mm×˜6 mm×˜32 mm. After cutting, the four surfacesof the tensile specimen parallel to the axial direction weremechanically polished with 600-grit silicon carbide papers to removeirregularities. Tensile testing along the specimen axial direction wasperformed to evaluate the tensile strength of the bonding interface,using an MTS810 system with hydraulic grips. X-ray diffraction (XRD)patterns from fractured bonding surfaces were collected on a RigakuMiniFlex X-ray diffractometer using Cu Kα radiation.

Tensile testing was performed on bonded Cu two-coupon assemblies. Duringtensile testing, the two ends of the specimen were gripped by thehydraulic grips. The gripped sections were ˜10 mm long, leaving anun-gripped section ˜12 mm long, with the bonding interface in themiddle. As the specimen extension increased, the tensile stressincreased to a maximum, followed by breaking at the bonding interfaceand a rapid drop in stress. The maximum tensile stress observed on thestress-extension curve is a measure of the tensile bond strength.

FIG. 15 depicts measured bond strength as a function of the appliedbonding pressure. For this series of measurements, all bonding runs wereperformed at ˜580° C. with an Al film thickness of ˜25 μm. In all cases,clean breaks occurred at the bonding interface. At the lowest pressureof ˜1 MPa, measured bond strength ranged from 33 to 45 MPa. Nosignificant difference was seen in tests performed on specimens bondedinstead at pressures of ˜2 or ˜4 MPa. The maximum strength measured was˜48 MPa.

FIG. 16 depicts measured bond strength as a function of the thickness ofthe Al film at the bonding interface. For this series of measurements,all bonding runs were performed at ˜580° C. at an applied pressure of ˜4MPa. At the lowest Al film thickness of ˜10 μm, measured bond strengthranged from 39 to 52 MPa. No gross differences were evident in testsperformed on specimens bonded with Al films with thickness of ˜25 or ˜38μm. Measured bond strength values ranged from 33 to 46 MPa. Data shownin FIGS. 15 and 16 indicated little variation in bond quality within therange of applied pressure and Al film thickness tested.

FIG. 17 depicts three XRD patterns obtained from fracture surfacesbonded at Al film thicknesses of 10, 25, and 38 μm, respectively. Majordiffraction peaks within all three patterns can be assigned to either anfcc Cu phase, or to a fcc Al phase, indicating the presence of thesephases in the interface region after bonding. The lattice parameters forthe Cu phase were 3.663 Å, 3.658 Å, and 3.667 Å at Al foil thicknessesof 10, 25, and 38 μm, respectively. In comparison, the lattice parameterfor the starting Cu coupon was measured at 3.618 Å. The correspondinglattice parameters for the Al phase were 4.102 Å, 4.103 Å, and 4.111 Å,respectively. In comparison, the lattice parameter for elemental Al is4.05 Å. The respective lattice parameter increases over those ofelemental Cu and Al suggest the possibility that there may have beensome dissolution of Al into Cu and vice versa in the interface region.Besides those indexed to fcc Cu and fcc Al phases, other diffractionpeaks were seen in all three XRD patterns, indicating the formation ofadditional Al—Cu compounds within the interface region during bonding.According to the standard Al—Cu phase diagram, a single eutectic shouldexist between the fcc Al phase and the θ-Al₂Cu phase. If bondingoccurred solely via the eutectic mechanism, only the θ-Al₂Cu phase wouldbe expected to be present in the interface region. Not all theadditional diffraction peaks present within the three XRD patterns canbe assigned to θ-Al₂Cu, however, suggesting the presence of Al—Cucompounds other than θ-Al₂Cu in the interface region and suggesting thatbonding may have occurred via a combination of both eutectic anddiffusional mechanisms.

Assembly of Metal-Based Microchannel Devices

Another aspect of this invention pertains to the fluidic transitionsfrom the microchannel arrays to larger scale plena. To create functionalmetal-based microchannel devices, for example for heat exchangerapplications, it is desirable to have techniques to fabricateunobstructed fluidic transitions from microchannel arrays to largerscale plena. The larger scale plena are used to provide fluidic inletand outlet connections to the “outside world.”

Factors in designing microchannel-to-large-plenum transitions are easeof operation, and the parallel creation of many transitions at once.Serial subtractive fabrication techniques are not well-suited for thispurpose. For example, micromilling would involve contact of a millingtool with formed microchannels, and could cause deformation at themicrochannel-to-plenum transitions, leading to partial or completeblockage. This mechanical machining also demands small-scale tooling,perhaps on the order of 100 micron or even smaller. Furthermore, thiskind of serial operation creates only one transition at a time, and isvery time consuming for a large number of connections for microchannelarrays. It is therefore preferred that fabrication protocols shouldinvolve parallel forming or machining. Non-contact machining methods arealso preferred.

EXAMPLE 11

Creating microchannel-to-large-plenum fluidic transitions by moldingreplication. FIG. 18 depicts an example of a mold insert design suitablefor creating a microchannel-to-large-plenum fluidic transition bymolding. In this embodiment, a long meandering microchannel terminatesat a triangular opening, which can widen into a large scale plenum. Themicrochannel structure shown in FIG. 18 can be created by moldingreplication.

EXAMPLE 12

Creating microchannel-to-large-plenum fluidic transitions by non-contactmachining methods. FIGS. 19 and 20 depict an example ofmultiple-microchannels-to-large-plenum fluidic transitions created in Cuby μEDM. An array of straight microchannels created by moldingreplication was connected to one supply channel and to one drain plenumusing μEDM. Flat stainless steel sheets with a thickness of 1100 μm wereused as blade electrodes for μEDM. A single cut with the steel bladeelectrode formed the fluid supply channel, while six consecutive cutswith partial overlaps formed the fluid drain plenum. All cuts wereperpendicular to the microchannel array. The depth of the cuts was thesame for all cuts on both the supply and drain sides.

As shown in FIG. 19 for one Cu coupon, mechanical drilling was used tomake two through holes to connect to the supply channel, and threethrough holes within the drain plenum. The through holes in the drainplenum were placed symmetrically with respect to the microchannel array.All through holes were mechanically tapped from the coupon's far side,away from the microchannel array, to allow external fluid connectionsusing plastic adapters. The 6.4 mm coupon thickness sufficed toaccommodate the taps. FIG. 20 provides a more detailed view of thefluidic supply connections on the Cu coupon from one tapped hole to themicrochannels. The supply channels had the mottled surface morphologytypical of structures cut by μEDM. The entrances were unobstructed.

Alternatively, the microchannel-to-plenum transitions shown in FIGS. 19and 20 can be formed by molding with an appropriate insert geometrydesign.

Improving Heat Exchange Efficiency with Metal-Based Microchannels HavingSubstantial Surface Roughness

A further aspect of the invention pertains to the creation ofmicrochannel heat exchangers with engineered surface roughness withinthe microchannels. Surprisingly, we found that surface roughness withinmicrochannels, for example on the order of a few microns to several tensof microns, substantially increases convective heat transfer performanceof the entire device as compared to an otherwise similar device withsmoother surfaces, i.e., one having a roughness less than a few microns.We further found that molding techniques are well adapted to replicatesuch surface roughness in metal-based microchannels. For example, thesurface roughness of the refractory metal or alloy mold insert can bealtered through the μEDM and ECP process control, for example bycontrolling the current density or etch time. Such surface roughness onthe mold insert is conveyed with high fidelity through the moldingprocess onto the metal substrate being molded. By contrast, such surfaceroughness would usually not be seen in microchannel structures made byconventional “semiconductor/IC-type” processing methods. With the novelmolding replication technique, reproducible roughness within metal-basedmicrochannels becomes fast and inexpensive.

We have discovered, quite unexpectedly, that this surface roughnesssubstantially enhances microchannel heat exchanger performance. Our datasuggested that surface roughness within the microchannels promotes fluidflow mixing to a surprising degree, which consequently increases theconvective heat transfer coefficient as compared to similarmicrochannels with smooth surfaces. Such heat transfer enhancementsexist over a large range of flow rates or Reynolds numbers.

EXAMPLES 13 AND 14

Enhancing heat transfer performance in Cu- and Al-based, single-layer,microchannel devices with increased surface roughness. We preparedbonded Cu and Al microchannel devices, and attached polymer externalfluid adaptors. (Not shown; see FIG. 26 of priority application61/020,789.) The internal microchannel array configurations within thesedevices was the same as that shown in FIGS. 19 and 20. FIG. 21 depicts ahigh-magnification scanning electron micrograph of a portion of thereplicated Cu microchannel array. The Cu microchannels have verticalsidewalls and sharp sidewall-to-bottom transitions. Elevated surfaceroughness is clearly visible on the sidewall and bottom of allmicrochannels. Generally similar observations were made in the Almicrochannel device (data not shown).

The surface roughness within the molded microchannels was quantitativelyevaluated by optical profilometry, expressed as peak-to-valley roughnessRz. The average Rz values were 11.8 μm and 8.2 μm for the bottomsurfaces of Cu and Al microchannels, respectively. Surface roughness ofthe microchannel sidewalls was somewhat smaller, on the order of 5 μm.The observed Rz values, on the order of 10 μm, substantially exceededwhat is typically obtained from micromilling (typically 1 μm or less).

We compared our measured heat transfer rates with some that have beenreported in the literature. (Data not shown; see FIG. 28 of priorityapplication 61/020,789.) Measured heat transfer coefficients, h, wereconverted to dimensionless Nusselt numbers, Nu, based on the averagehydraulic diameter of the microchannels, D_(h),

${{Nu} = \frac{{hD}_{h}}{K_{f}}},$

where K_(f) is the thermal conductivity of the fluid. Values of Nu areplotted versus the Reynolds number, Re, which represents a dimensionlessaverage fluid velocity through the microchannels, V. The value of Re isdefined as

${Re} = {\frac{\rho \; \overset{\_}{V}D_{h}}{\mu}.}$

Both the fluid density ρ and the viscosity μ (in this case for liquidwater) may depend on temperature.

In the range 500<Re<2250, good agreement existed between Nusselt numbersmeasured from the Cu and the Al MHE specimens. For Re>2500, data fromthe Cu and the Al MHE specimens began to diverge, with Nu values fromthe Cu specimen exceeding those from the Al specimen.

We compared our measurements to data taken from Lee et al., Int. J. HeatMass Transfer 48(9), 1688-1704 (2005) for machined Cu microchannels withsmooth surfaces; the data set of Lee et al. were taken frommicrochannels with D_(h)=318 μm, and extended over a smaller range ofRe, from ˜500 to ˜2500. Our measured Nu values were substantially andsignificantly higher than those of Lee et al. over much of the range500<Re<2500. We also compared our measured values to some conventionalNusselt number correlations. Correlations for fully developed laminarflow and the Sieder-Tate correlation for simultaneously developinglaminar flow were calculated using dimensions corresponding to the Cumicrochannel device. The Gnielinski correlation for transitional andfully developed turbulent flows was also calculated. Our data from thenovel Cu and Al microchannel devices exhibited trends generally similarto those predicted by the conventional correlations, but with highervalues of Nu. (Data not shown; see FIG. 28 of priority application61/020,789.)

The data obtained from the Cu and Al microchannel devices, as well asthe data of Lee et al., showed Nusselt number increasing with increasingReynolds number. At the same Re values, our data showed higher Nu valuesas compared to those of Lee et al. Direct visualization of flow withinthe microchannel arrays would be impractical for the assembled, opaqueCu and Al microchannel devices. Nonetheless, it appears that the surfaceroughness within the microchannel arrays resulting from the moldingreplication process is responsible for the observed higher Nu values.The increased surface roughness within the microchannel arrays may, forexample, lead to increased cross-wise flow mixing, resulting in higherheat transfer as compared to that in smoother channels.

Accurate measurements of the solid-to-fluid heat transfer rate requirean accurate estimate of the solid wall temperature of the microchannelarray. Relatively large temperature gradients can be induced within thebody of the metal-based MHEs during constant heat flux testing, makingthe estimate of solid wall temperature less reliable. Therefore, analternative, constant solid surface temperature testing configurationwas adopted to measure heat transfer rates more accurately. Higher Nuvalues were obtained as compared to the uncorrected results, approaching40 at Re of ˜3000. The maximum uncertainty for Nu values was ˜16%.

We compared known Nusselt number correlations to our experimental data.The experimental Nu values significantly exceeded the Hausen andSieder-Tate correlation values at 250<Re<1500.

For turbulent flow, the Dittus-Boelter correlation and the Petukhovcorrelation were used in the Reynolds number range 2000<Re<3000. Thesetwo correlations yielded nearly identical Nu values over this Reynoldsnumber range. Our measured Nu values substantially exceeded theDittus-Boelter and Petukhov correlation values. When corrections weremade to account for a surface roughness of ˜5 μm, the correlationsbetter matched the observed Nu data. These trends illustrate the need toconsider surface roughness and entrance length effects in analyzing flowand heat transfer data, and further demonstrate that engineering surfaceroughness into microchannel surfaces can be an effective means toincrease heat transfer efficiency.

The usefulness of heat transfer devices in accordance with the presentinvention is illustrated in FIG. 22. Initially, heaters attached to theprototype Cu microchannel device were turned on and the device wasallowed to reach a temperature of about 100° C. Keeping the same powerinput, about 20 W, in the heaters, room temperature water (˜28° C.) wassuddenly introduced into the Cu microchannel device with a fixedpressure drop ΔP from the outlet to the inlet. The device's surfacetemperature was monitored in real time using an infrared (IR) camera.FIG. 22 shows the change in surface temperature of the Cu microchanneldevice as a function of time, both before and after introduction ofwater into the specimen at time t=10 sec. The surface temperaturedropped more than 50° C. within the first 5 seconds, followed by a moregradual decrease to ˜30° C. over the next 60 seconds. The initial,faster drop in temperature was attributed to drawing heat from the Cu.The second and more gradual drop in temperature was attributed todrawing heat from the surrounding insulation material. A fit was made ofthe measured surface temperature versus time, assuming a compoundexponential decay with two distinct time constants τ₁ and τ₂. Consistentwith expectation, τ₁ was on the order of a few seconds while τ₂ was onthe order of a few tens of seconds. FIG. 23 shows values of the fittedtime constant τ₁ for both the Cu and Al microchannel devices, obtainedat different values of ΔP and initial specimen temperature T₁. As T₁varied from 70 to 100° C., the values of τ₁ showed little variation.However, τ₁ decreased with increasing ΔP or water flow rate. τ₁ was inthe range of 1 to 2 s for the Cu device, and 2 to 2.5 s for the Aldevice. This very rapid cooling demonstrated the advantages ofincorporating high thermal conductivity metals, such as Cu and Al, intoheat exchangers.

EXAMPLE 15

Two-layer, Cu-based, microchannel devices incorporating heatingcartridges: instant water heater prototype. A process generally similarto that described above was used to manufacture Cu-based, two-layermicrochannel, “instant” water heater prototypes. Cu blocks containingholes for accommodating cylindrical, electric cartridge heaters, andholes/plena for fluidic connections were made by conventional machining.Arrays of parallel microchannels were replicated in Cu coupons bymolding. Surfaces of Cu blocks and coupons were polished with 1200-gritsilicon carbide papers prior to bonding. The prototype assemblycomprised one such Cu block placed between two Cu coupons, eachcontaining a parallel array of replicated microchannels. One 10μm-thick, free-standing Al thin film was inserted at each coupon/blockinterface. The entire coupon/block/coupon assembly was placed on top ofthe bottom heating station. After evacuation, both heating stations wereheated above 500° C., and the upper heating station was placed incontact with the assembly. An increasing compression force was appliedto the assembly at a constant loading rate of 300 N/min. The force washeld constant for 15 min once the compression force reached ˜3000 N,corresponding to an average applied pressure of ˜3 MPa. The finalbonding temperature for the coupon/block/coupon assemblies was held at˜580° C. After the constant force hold, the linear actuator waswithdrawn from the assembly and the system was cooled down.

A photograph of a breakdown of a prototype instant water heater assemblyis shown in FIG. 24. There were two Cu coupons, each containing one setof parallel rectangular microchannels ˜15 mm long, ˜150 μm wide, ˜400 μmdeep, covering a total area of ˜15 mm×15 mm, together with a custom-madeCu heater block with overall dimensions of 43 mm×43 mm×15 mm. On the topheater block surface, two plena were machined to connect to the two endsof the microchannel array on the Cu coupon. The distance between the twoplena was ˜12.8 mm. The bottom heater block surface had an identicalconfiguration (not visible in FIG. 24). A hole in each plenum, ˜7.5 mmdiameter (labeled “A” in FIG. 24), was machined through the entireheater block to connect the two plena on the top and bottom heater blocksurfaces. An additional hole on each side of the heater block wasmachined perpendicular to hole A (labeled B in FIG. 24), for externalwater connections. Four parallel holes (labeled C in FIG. 24), each witha diameter ˜6.4 mm and a length ˜41 mm, were drilled parallel to hole B,and were used to house four cylindrical cartridge, 180 W,electrical-resistance heaters.

A testing apparatus was designed and built to evaluate the heat transfercharacteristics of assembled Cu instant water heater prototype. Theapparatus comprised three principal sections: water supply section, testsection, and data acquisition section. The water supply sectioncomprised a pressure-regulated water storage tank, which supplied waterto the specimen at a constant pressure for a smooth and stable flowthrough the microchannels at low flow rates. A valve downstream of thetank exit was used for fine adjustments to the flow rate. An Instrunetdata acquisition system interfaced to a PC was used to collectthermocouple readings.

Water flow through the assembled Cu instant water heater prototype, andheat transfer from the cartridge heaters to water were measured. Thetotal pressure drop across the inlet and exit fluid connections wasmeasured with a Dywer digital manometer with a minimum reading of 690 Pa(˜0.1 psi). The rates of water flow through the microchannel arrays inthe prototype were measured as a function of the associated pressuredrop. The flow rate increased monotonically with increasing pressuredrop across the prototype, and reached 1.5 liter/min at a pressure dropof ˜0.48 MPa (˜70 psi).

Thermocouples were inserted into both the inlet and outlet tubes withT-fittings, and were sealed with epoxy cement. Additional thermocoupleswere placed on the top and bottom surfaces of the prototype, as well ason the side surface closest to the heaters. Because none of thethermocouples were placed within the microchannels, whether the waterflow within the microchannels is laminar or turbulent should not affectthe temperature measurements. The entire assembly was then encasedwithin PVC insulation, with holes drilled into the PVC to allow for thefluid inlet, outlet, and pressure meter tube connections. FIG. 25depicts the (outlet-inlet) difference in water temperature as a functionof water flow rate through the prototype, measured at two differentheater input powers. At the higher input power of ˜710 W, the watertemperature increased >7° C. at a flow rate of ˜1.4 liter/min and >25°C. at a flow rate of ˜0.4 liter/min. The efficiency of heat transfer,defined as the ratio of power gained by water to total heater inputpower, is shown in FIG. 26 as a function of the water flow rate. Theefficiency was very high in all cases, ranging from just under 98%, to amaximum of 99.7% at higher flow rates. At higher flow rates, the lowerprototype body temperature resulted in lower heat loss to the PVCinsulation and therefore higher overall heat transfer efficiency.

Miscellaneous

Those of skill in the art will recognize that various modifications maybe made to the embodiments described above, while staying within thescope of the present inventions. Among the possible modifications andalternative embodiments are those described above and below.

A metal-based microchannel heat exchange device may be formed of avariety of metals, including aluminum, aluminum-based alloys, copper,copper-based alloys, nickel, or nickel-based alloys, such asnickel-titanium alloys.

The microchannel arrays may be straight, curved, or profiled, in any orall dimensions.

The microchannel arrays may be formed from multiple metal sheets, withmultiple bonding interfaces, or multiple connections from microchannelarrays to fluid inlet and outlet plena. The microchannels preferablyhave elevated surface roughness to improve heat exchange, for example inthe range about 1 μm≦Rz≦20 μm, preferably the range about 3 μm≦Rz≦15 μm,most preferably about 10 μm. The insert surface roughness may becontrolled, for example, by altering electrochemical polishingconditions.

The microchannel arrays are preferably made by microscale compressionmolding using a microscale mold insert made of a refractory metal oralloy. Among the materials that may be used for the refractory moldinsert are the following: Ta, W, Mo, Nb; their respective binary,ternary, and quaternary alloys, with or without metalloid elementadditions such as C, B, Si; transition metals and alloys, such as Hf,Zr, Ti, V, Cr; their respective binary, ternary, and quaternary alloys,with or without metalloid element additions such as C, B, Si; allclasses of Fe-based tool steels, including M-series, T-series, andH-series tool steels; a Ni-based alloy or superalloy, for example one ofthe Inconel series of Ni alloys; or a refractory ceramic, including ametal carbide such as TaC, WC, MoC, TiC, NbC, pseudo-binary alloys ofmetal carbides, such as TaC—WC, or a metal nitride such as TaN, WN, MoN,TiN, NbN, or pseudo-binary alloys of metal nitrides, such as TaN—WN; ordiamond. The inserts may be made, for example, by electrical dischargemachining, or by micro-electrical discharge machining usinglithographically patterned electrodes. The inserts may optionally beconformally coated with a suitable ceramic, carbon, or hydrocarboncoating.

The bonding interfaces may occur between metal sheets containingmicrochannel arrays, or between metal sheets containing microchannelarrays and solid plates, or between metal sheets containing microchannelarrays and perforated plates. The thin film intermediate bonding layermay itself have a eutectic or near-eutectic composition; or it may be aeutectic precursor that will form a eutectic or near-eutecticcomposition when heated in contact with the adjacent metal piece(s). Theintermediate bonding layer may, for example, comprise a free-standingthin film (eutectic precursors), e.g., Cu, Zn, Al, Mg, Sn, Ga, In, orNi; or it may comprise a eutectic or near-eutectic nanocomposite layer,e.g., Al—Ge, Al—Si, Al—Mg, Al—Sn, Al—Ga, Au—Si, Cu—Al, Al—Zn, Sn—In,Cu—In, Au—In, Ag—In, Ag—Sn, Cr—Sn, Cu—Sn, Au—Sn, and binary, ternary, orquaternary mixtures of any of the above. The individual domains withinthe eutectic bonding layer are preferably primarily in the range fromabout 100 nm or smaller to about 400 nm. Alternatively, a free standingthin metal film may be used as a eutectic precursor, including forexample Al, Sn, Zn, Cu, Ni, or their alloys. The individual bondinglayer thickness is preferably from about 0.3 to about 10 μm. The thinfilm eutectic or near-eutectic bonding layer may, for example befabricated by direct physical or chemical vapor phase deposition ontothe pieces to be bonded; or a free-standing thin film may be fabricatedby metallurgical means and inserted between the pieces to be bonded; orother techniques otherwise known in the art may be used, such as sputterdeposition or co-deposition, co-evaporation, or e-beam co-evaporation.The pieces are heated to an appropriate temperature to ensure properquality of bonding, without excessive deformation, by heating to atemperature at or just above the eutectic point; for example from about450° C. to about 550° C. in the case of aluminum or aluminum-containingalloys, or from about 500° C. to about 600° C. in the case of copper orcopper-containing alloys. Preferably pressure is applied during theheating step to promote bonding between the thin film layer and themetal workpiece, for example, from about 0.5 to about 5 MPa, or higher.

Among the materials that may be used for the low-melting metal to formthe heat exchanger are the following: Aluminum and aluminum-basedalloys, such as the 1 xxx series of Al alloys, 2xxx series of Al alloys,3xxx series of Al alloys, 5xxx series of Al alloys, 6xxx series of Alalloys, and 7xxx series of Al alloys; Copper and copper-based alloys,e.g. the C1xxxx series of Cu alloys, C2xxxx series of Cu alloys, C5xxxxseries of Cu alloys, and C7xxxx series of Cu alloys; Nickel andnickel-based alloys, e.g. Ni—Ti alloys, Ni—Cu alloys, Ni—Al alloys; Zincand zinc-based alloys; and Magnesium and magnesium-based alloys.

Preferred ranges for various dimensions and other numerical values innovel heat exchangers in accordance with the present invention are: (a)Microchannel length: from about 100 μm upwards (no upper limit inlength; for example, a meandering channel could be several meters intotal length). (b) Microchannel width: from about 30 μm minimum to about2000 μm, preferably from about 30 μm to about 1000 μm. (c) Microchanneldepth: from about 30 μm to about 2000 μm, preferably from about 30 μm toabout 1000 μm. (d) Microchannel cross-sectional aspect ratio: from about0.03 to about 35. (e) Number of microchannels in heat exchanger: from 1(e.g., one long meandering channel) to 1000 or 10,000 (e.g., a totaldevice width of about one meter). (f) Overall dimensions of metal-basedheat exchangers: from about 5 mm×5 mm×1.5 mm (e.g., aone-microchannel-layer device for cooling a single “hot-spot”); to about1000 mm×100 mm×100 mm (e.g., a multiple-microchannel-layer device).

Definitions. The “length” of a microchannel is defined as its totaldistance measured along the direction in which fluid will generally tendto flow through the microchannel, measured along the path of that fluidflow. The “width” and “depth” of a microchannel are distances measuredperpendicular (or approximately perpendicular) both to each other and tothe length of the microchannel. There is no preferred direction definedas “width,” nor as “depth,” but each may be taken in a convenientdirection, consistent with the preceding definitions. The use of theterms “length,” “width,” and “depth” should not be construed to implythat the microchannel must assume any particular shape. As a few of manypossible examples, a cross-section of a microchannel may be square,circular, rectangular, or elliptical; and the microchannel itself may bestraight, curved, spiral, sinusoidal, serpentine, a racetrack, etc. A“homogeneous” metal layer or metal component is one that is essentiallyuniform throughout, except perhaps at a surface or boundary where it maybe brazed or joined to another layer or component. More specifically, acomponent that contains separate, multiple, internal layers of metalinterspersed (or brazed together) with separate, multiple, internaleutectic layers is not considered to be “homogeneous” within the scopeof this definition. Different “homogenous” components in the same deviceneed not necessarily have the same composition, although in many casesit will be preferred that their compositions should be the same. Amicrochannel is considered to be “enclosed entirely” by specifiedcomponents (such as by two homogeneous metal layers and a eutecticlayer) if it is enclosed and bounded by the specified components—and byno other components—along essentially the entire length of themicrochannel; with possible exceptions at (and only at) themicrochannel's fluid inlet(s) and fluid outlet(s). At the inlet(s) andoutlet(s), the microchannel may optionally be open rather than closed;and at the inlet(s) and outlet(s) the microchannel may optionallyconnect to or be bounded by other component(s).

The complete disclosures of all references cited in this specificationare hereby incorporated by reference; including, by way of example andnot limitation, the entire disclosure of priority U.S. provisionalapplication 61/020,789, filed 14 Jan. 2008. In the event of an otherwiseirreconcilable conflict, however, the present specification shallcontrol.

1. A process for making a metal microchannel heat exchanger, said process comprising the steps of: (a) forming one or more open microchannels on a surface of a first homogeneous metal piece, wherein at least one of the microchannels has a width between about 30 μm and about 1000 μm, and a depth between about 30 μm and about 1000 μm; (b) providing a second homogeneous metal piece that, when bonded to the first metal piece, will convert one or more open microchannels on the first metal piece into one or more closed microchannels, wherein the one or more closed microchannels are adapted to transport liquid without substantial leakage; (c) providing a eutectic layer or eutectic precursor layer at one or more of the following locations: a surface of the first metal piece, a surface of the second metal piece, or between the first and second metal pieces; (d) simultaneously applying pressure to and heating the first and second metal pieces, wherein: (i) the pressure pushes the first and second metal pieces toward each other, with the eutectic layer or eutectic precursor layer between the first and second metal pieces; (ii) the pieces are heated to a temperature at which the eutectic layer or eutectic precursor layer melts, or at which the eutectic layer or eutectic precursor layer interacts with the metal pieces to form a molten eutectic composition between the first and second metal pieces; and (iii) the temperature to which the metal pieces are heated is sufficiently below the melting temperature of the first and second metal pieces that no substantial deformation of the one or more microchannels occurs; (e) cooling the first and second metal pieces to a temperature substantially below the eutectic melting temperature, while maintaining the pressure during at least a portion of said cooling; such that the first and second metal pieces fuse together; such that the one or more open microchannels are converted into one or more closed microchannels, wherein the one or more closed microchannels are adapted to transport liquid without substantial leakage; and wherein no substantial blockage of the one or more closed microchannels occurs as a result of said heating, applying pressure, and cooling; and wherein: (f) the one or more closed microchannels are enclosed entirely by the fused first and second metal pieces and eutectic layer; whereby the fused first and second homogeneous pieces and eutectic layer, together with the enclosed one or more closed microchannels, form a microchannel heat exchanger.
 2. A process as in claim 1, wherein the heat exchanger is capable of withstanding an internal pressure in the one or more closed microchannels of 100 atmospheres or greater.
 3. A process as in claim 1, wherein said microchannel-forming step comprises compression molding of one or both metal pieces with a refractory metal mold insert.
 4. A process as in claim 1, wherein at least one of the closed microchannels has a surface roughness between about 3 μm and about 15 μm.
 5. A metal microchannel heat exchanger produced by the process of claim
 1. 6. A metal microchannel heat exchanger comprising one or more closed microchannels; wherein at least one of said microchannels: (a) is enclosed entirely by a first homogeneous metal piece, a second homogeneous metal piece, and a eutectic layer; wherein said first and second homogeneous metal pieces are brazed to one another by said eutectic layer; (b) has a width between about 30 μm and about 1000 μm, and a depth between about 30 μm and about 1000 μm; and (c) is adapted to transport liquid without substantial leakage.
 7. A heat exchanger as in claim 6, wherein said heat exchanger is capable of withstanding an internal pressure in said one or more closed microchannels of 100 atmospheres or greater.
 8. A heat exchanger as in claim 6, wherein at least one of said closed microchannels has a surface roughness between about 3 μm and about 15 μm. 